The effects of salbutamol 0.225 mg/kg given systemically on lung function in infants have been measured by whole-body plethysmography in 60 children with broncho-pulmonary diseases (24 after respiratory distress syndrome, 21 with wheezy bronchitis and 15 with cystic fibrosis). A therapeutic action was demonstrated in 74% of the tests, taking into account changes in end-expiratory resting level (state of inflation) and associated changes in airway resistance. There was a significant decrease in thoracic gas volume as an estimate of pulmonary hyperinflation, which was due to improved alveolar ventilation and to a consequential decrease in end-expiratory resting level. The improvement in airway resistance, as an estimate of airway obstruction, reflects a substantial relaxation of bronchial smooth muscle increasing the diameter of the airways. The extent to which similar results may be achieved by topical administration of nebulised beta agonists remains to be determined.